3g is a signal to get online from anywhere a cellphone can get a signal.
wifi is a signal to get online from a computer/network nearby.
EX: for 3g you can pick up a signal because someone has a AT&T cellphone.
for wifi you can pick a signal sitting at McDonalds.
I actually just sort of learned the difference the other day.
My KK3 has WIFI only and i kind of like it that way.